1. Field of the Invention
This invention relates to a device for detecting an applied voltage to a power convertor, and particularly to a device for detecting an applied voltage to a high voltage thyristor convertor consisting of a plurality of semiconductor power control rectification elements such as thyristors that are connected in series.
2. Description of the Prior Art
Generally, a thyristor convertor used in high voltage circuits is constituted by a plurality of series thyristors in order to reduce the shared voltage per element and to obtain a lower withstand voltage. However, practical characteristics of thyristors are not uniform; turn-on time and turn-off time may vary from thyristor to thyristor. Therefore, when commutation is made from turn-on to turn-off, an overvoltage is applied to a thyristor or thyristors having a short turn-off time, and such thyristors may be damaged. When commutation is made from turn-off to turn-on, an over-voltage is also applied to a thyristor or thyristors having bad turn-off characteristics, and such thyristors also may be damaged.
To prevent damage to thyristors due to the variation of the element characteristics, an applied voltage detecting device has been employed which detects a forward voltage and a backward voltage applied to each series thyristor and adjusts the turn-on timing of turn-on control.
An example of the applied voltage detecting devices of this kind is described, for example, in FIG. 5 of the article entieled PERFORMANCE OF HIGH VOLTAGE LIGHT-TRIGGERED THYRISTOR VALVE", IEEE Transactions on Power Apparatus and Systems, Vol. PAS-102, No. 8, August, 1983.
In the prior art reference described above, an optical thyristor LED and an optical guide LG are disposed for each of the forward and backward voltages in order to detect two kinds of voltages applied to the thyristor.
If a plurality of applied voltage detecting devices are disposed for each of the thyristors, the number of components is great and the device is unavoidably large and complex.